MAN: Document memcache_timeout=0 meaning
    
    Document that by setting memcache_timeout to 0 the in-memoory cache
    will be disabled.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3496
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>